std::expected<T,E>::operator->, std::expected<T,E>::operator*

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Diagnostics library

Memory management library

Metaprogramming library (C++11)

General utilities library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library (C++17)

Concurrency support library (C++11)

Execution control library (C++26)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Program utilities Variadic functions initializer_list (C++11) is_constant_evaluated (C++20) is_within_lifetime (C++26) source_location (C++20) Coroutine support (C++20) Contract support (C++26) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Function objects Bit manipulation (C++20) bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23)

 

std::expected

Member functions

expected::expected

expected::~expected

expected::operator=

Observers

expected::operator->expected::operator*

expected::operator boolexpected::has_value

expected::value

expected::error

expected::value_or

expected::error_or

Monadic operations

expected::and_then

expected::or_else

expected::transform

expected::transform_error

Modifiers

expected::emplace

expected::swap

Non-member functions

operator==(std::expected)

swap(std::expected)

Helper classes

unexpected

bad_expected_access

unexpect_tunexpect

 

Primary template
constexpr const T* operator->() const noexcept;	(1)	(since C++23)
constexpr T* operator->() noexcept;	(2)	(since C++23)
constexpr const T& operator*() const& noexcept;	(3)	(since C++23)
constexpr T& operator*() & noexcept;	(4)	(since C++23)
constexpr const T&& operator*() const&& noexcept;	(5)	(since C++23)
constexpr T&& operator*() && noexcept;	(6)	(since C++23)
void partial specialization
constexpr void operator*() const noexcept;	(7)	(since C++23)

See More

Accesses the expected value contained in *this.

1,2) Returns a pointer to the expected value.

3-6) Returns a reference to the expected value.

7) Returns nothing.

If has_value() is false, the behavior is undefined.	(until C++26)
If has_value() is false: If the implementation is hardened, a contract violation occurs. Moreover, if the contract-violation handler returns under “observe” evaluation semantic, the behavior is undefined. If the implementation is not hardened, the behavior is undefined.	(since C++26)

Return value

1,2) std::addressof(val )

3,4) val

5,6) std::move(val )

Notes

These operators do not check whether the optional represents an expected value, users can do so manually by using has_value() or operator bool(). Alternatively, if checked access is needed, value() or value_or() may be used.

Example

Run this code

#include <cassert>
#include <expected>
#include <iomanip>
#include <iostream>
#include <string>

int main()
{
    using namespace std::string_literals;
    
    std::expected<int, std::string> ex1 = 6;
    assert(*ex1 == 6);
    
    *ex1 = 9;
    assert(*ex1 == 9);
    
    // *ex1 = "error"s; // error, ex1 contains an expected value of type int
    ex1 = std::unexpected("error"s);
    // *ex1 = 13; // UB, ex1 contains an unexpected value
    assert(ex1.value_or(42) == 42);
    
    std::expected<std::string, bool> ex2 = "Moon"s;
    std::cout << "ex2: " << std::quoted(*ex2) << ", size: " << ex2->size() << '\n';
    
    // You can "take" the expected value by calling operator* on an std::expected rvalue
    
    auto taken = *std::move(ex2);
    std::cout << "taken " << std::quoted(taken) << "\n"
                 "ex2: " << std::quoted(*ex2) << ", size: " << ex2->size() << '\n';
}

Possible output:

ex2: "Moon", size: 4
taken "Moon"
ex2: "", size: 0

See also

value	returns the expected value (public member function)
value_or	returns the expected value if present, another value otherwise (public member function)
operator boolhas_value	checks whether the object contains an expected value (public member function)
error	returns the unexpected value (public member function)